sewage treatment plants

bacterium biology biomass circle clean cracked damp dirt dirty ecological ecology engineering environment environmental excremental excretion factory filter filtration flow garbage global industrial industry nasty natural not pipe pipes plant plastic pollute pollution pool processing protection pumps purification purify purity recycle recycling reduce reflection refuse repulsive responsible rot rubbish rural sanitary sanitation sediment sedimentation settling sewage sludge smell solids spokes station system tank trash treatment unhygienic valve warming waste wastewater wastrel water